1. Field of the Invention
The present invention relates to a toner for various recording methods, such as those based on electrophotography, electrostatic recording, magnetic recording and toner jet recording, more particularly to a toner useful for copiers, printers and facsimiles in which a toner image is formed on an electrostatic latent image carrier and the toner image is transferred onto a medium to form the final image.
2. Related Background Art
A number of electrophotographic methods have been proposed. They generally use a photoconductive material to form an electrical latent image on an image carrier (photosensitive member) by various methods, which is developed by a toner into a visible image, transferred, as required, onto transfer medium such as paper or other media, and fixed into the toner image on the above medium by heat, pressure or the like, for copying the image.
Methods to visualize an electrical latent image include cascade, magnetic-brush and pressurization development. Another method uses a magnetic toner, which is scattered by an electric field in a space between a photosensitive member and sleeve using a rotating sleeve with a magnetic pole at the center.
The one-component developing method, dispensing with carrier particles, e.g., glass beads and iron powders, which are necessary for the two-component method, reduces size and weight of the developing device itself. Moreover, the two-component developing method needs concentration of the toner in the carrier to be kept at a constant level, and hence a device which senses the toner concentration and supplies a required quantity of the toner. This further increases size and weight of the developing device. The one-component method does not need such device, and therefore, is more desirable also in this respect.
Recently, the printing devices have been mainly represented by LED and LBP printers, for which techniques are increasingly demanded to improve resolution from the traditional level of 240 or 300 dpi to 400, 600 and further to 800 dpi. The development method is also demanded to be more precise, accordingly. The copier is also becoming more functional, and advancing in the direction of digitalization. This direction is mainly associated with laser-aided formation of electrostatic images, and also demands development methods of higher resolution and precision, as is the case with printers. The toner particles, therefore, are becoming smaller. The smaller toner particles having a specific size distribution have been developed, as disclosed by Japanese Patent Application Laid-Open Nos. 1-112253, 1-191156, 2-214156, 2-284158, 3-181952, and 4-162048.
Recently, in particular, electrophotographic color-image forming devices are going into diversified applications, as they are used more widely, and are required to produce images of higher quality. It is demanded that common photographs, catalogs and maps are copied very finely and precisely to the finest portion, without forming any crushed or broken portion.
In the advanced electrophotographic image-forming devices using digital image signals, a latent image is formed by dots of a specific potential, assembled on a surface of latent image carrier or photosensitive member, where solid-color, halftone and line sections are expressed by changing dot density. This method, however, is liable to suffer problems related to color tone, because the toner particles may not be sufficiently confined in a dot, with some particles sticking out of the dot, making it difficult to secure the toner image corresponding to a dot density at a dark or bright portion of the digital latent image. When dot size is reduced to improve resolution and hence image quality, reproducibility of a latent image formed by fine dots tends to decrease, producing an image insufficient in resolution and particularly poor color tone at high-light sections, and lacking sharpness.
Moreover, the primary charging, transfer process, in which a photosensitive contact member produced by a primary charging and transfer process using conventional corona discharge is used, is becoming the major approach for environmental considerations.
The charging means using corona discharge, e.g., those named corotron and scorotron, generates a large quantity of ozone when negative corona is formed during the discharge. Therefore, the electrophotographic device must be equipped with an ozone-capturing filter, increasing device size and running cost. These problems involved in the corona charge methods cause the problems related to image quality, e.g., distorted image caused by reduced surface resistance of the photosensitive member as a result of contamination with, e.g., nitrogen oxides, and reduced memory of the photosensitive member resulting from ions remaining in the charging device while the electrophotographic device is out of service.
A new charging method was developed to solve the above problems, where a charging member such as roll or blade is brought into contact with the photosensitive member (this approach is hereinafter referred to as direct charging) to form a discharge explained by the Paschen's rule in a narrow space in the vicinity of the contact point. It is to minimize generation of ozone, and the related techniques are already disclosed by, e.g., Japanese Patent Application Laid-Open Nos. 57-178257, 56-104351, 58-40566, 58-139156, 58-150975, and 63-149669. Of these, the method which uses a charging roll as the charging member is more preferable for charging stability.
Direct charging generates a smaller quantity of ozone than corona discharging, conceivably because of different charging mechanisms on the photosensitive member surface, coming from different discharging regions. It is considered that the photosensitive member surface is charged in corona discharging with the ions, coming from dissociated molecules in air, in the discharging region, whereas it is charged in direct charging with a number of electrons reaching the surface by the multiplication effect of the electrons in the discharging region.
It is found, however, direct charging encounters with problems to be solved, different from those involved in corona charging.
Concretely, it is necessary to apply a voltage of at least certain threshold level to the charging member for direct charging to start, because it depends on discharge from a charging member to a member to be charged, such as photosensitive member. For example, when a charging roll is brought into contact with an OPC photosensitive member with a 25 .mu.m thick photosensitive layer, the photosensitive member starts to increase in surface potential when a voltage of at least around 640 V is applied to the charging member, the surface potential increasing linearly with voltage at an inclination of unity thereafter. This threshold voltage is hereinafter referred to as charge-starting voltage Vth. Therefore, in order to secure a potential Vd on the photosensitive member surface, the charging roll needs a higher DC voltage of Vd+Vth. It was difficult to keep a desired potential on the photosensitive member, because of resistance of the contacting charging member changing by external disturbances, e.g., changed environmental conditions.
In order to further uniformize charging, an AC charging method is proposed, as disclosed by Japanese Patent Application Laid-Open No. 63-149669, where an AC component having a voltage of at least twice as high as Vth between the peaks is added to the DC voltage corresponding to the desired Vd level, and the totaled voltage is applied to the contacting charging member. This is aimed at leveling the potential by the AC voltage, to significantly prevent potential on the member to be charged from external disturbances, such as environmental changes, because it tends to converge to Vd as the central voltage between the AC voltage peaks.
However, the above contacting charging device also basically depends on the mechanism of discharge from the charging member to the photosensitive member, and needs, as described above, a charging voltage higher than a potential on the photosensitive member surface. Addition of an AC voltage to uniformize charging involves new problems, such as generation of vibration and noise (hereinafter referred to as the AC charging noise) of the charging member and photosensitive member by the AC field, and deterioration or the like of the photosensitive member surface by additional discharging by the AC voltage.
It is inherently desirable to charge the photosensitive member only with DC voltage, to minimize generation of ozone. However, charging with DC voltage alone is more amenable to external disturbances, e.g., environmental changes, and contamination of the charging member to cause uneven charging.
Japanese Patent Application Laid-Open No. 2-123385 discloses a contacting transfer method, in which a toner image is produced by development on an electrostatic latent image carrier, and a medium to which the image is to be transferred is pressed to the carrier by a charged electroconductive roll, to transfer the image.
However, the above roll-aided transfer method, which dispenses with corona discharge, involves its own disadvantages, such as partial transfer failure (the so-called intermediate loose of transfer) resulting from the toner image being pressed while it is transferred from the photosensitive member to the medium because the transferring member directly comes into contact with the photosensitive member via the transferring member when it is transferred.
When the transfer toner remains on the photosensitive member after the toner image produced on the photosensitive member by development is transferred to the medium, as described above, it must be removed by the cleaning process and discarded in a spent toner container. The cleaning is effected by several methods, e.g., blade cleaning, fur brush cleaning and roller cleaning, each of which mechanically removes the residual toner by, e.g., scratching and discards it in a spent toner container. Pressing the cleaning member to the photosensitive member surface invariably causes problems, such as wear and reduced serviceability of the photosensitive member when the cleaning member is strongly pressed. The cleaning device increases size of the overall device, and bottlenecks size reduction. Moreover, it is ecologically desirable to develop a system which releases no spent toner and toner of higher transfer efficiency.
Techniques to dispense with the cleaner have been disclosed by, e.g., Japanese Patent Application Laid-Open Nos. 59-133573, 62-203182, 63-133179, 64-20587, 2-302772, 5-2289, 5-53482, and 5-61383. These prior arts, however, do not mentioned on desired toner compositions.
The image forming method which uses development/cleaning mechanism with virtually no cleaning device is liable to suffer various problems resulting from residual transfer toner directly passing through a space between the charging member and photosensitive member, e.g., contamination of the charging member and resultant uneven resistance of the member, and uneven charging which may cause extremely uneven concentration in the halftone image. The configuration invariably involves rubbing of the photosensitive member surface by the toner and toner carrier, resulting in deterioration of durability by, e.g., deteriorated toner and toner carrier surface, and deteriorated or worn photosensitive member surface, when the device is in service for extended periods. These problems cannot be sufficiently solved by the conventional techniques, and the techniques to improve characteristics related to development and durability and, at the same time, to prevent uneven charging are increasingly in demand.
Japanese Patent Application Laid-Open No. 61-279864 discloses a toner with specified shape factors SF-1 and SF-2. This prior art, however, does not completely mentioned on transfer, and needs further improvement, because of its insufficient transfer efficiency found by trace tests.
Japanese Patent Application Laid-Open No. 63-235953 discloses a magnetic toner, whose particles are made spherical by mechanical impact. However, it also needs further improvement, because of its still insufficient transfer efficiency.
Recently, color copiers, printers, facsimiles and the like, based on electrophotography, have been increasingly in demand.
Color toners are generally non-magnetic, because of insufficient tincture of a magnetic toner containing a magnetic material. When a magnetic toner is used as a black toner and non-magnetic one as color toner for a color electrophotographic device, the non-magnetic toner needs higher optimum transfer current level than the magnetic one. The magnetic toner transferred to a medium may return back to the latent image carrier (retransfer), when the electrophotographic device operates under conditions adjusted for the non-magnetic toner. Conversely, insufficient transfer of the non-magnetic toner may result, when the optimum conditions are adjusted for the black toner.
Therefore, simultaneous use of a magnetic and non-magnetic toner for image transfer causes problems which must be solved for low-cost production of a compact, light, color electrophotographic device producing images of high resolution and precision.
For the toner containing hydrotalcites, Japanese Patent No. 2,584,306 discloses an Mg/Al-based one, aimed at removal of NOx and the like from the photosensitive member surface. It is however insufficient, e.g., in charging stability. Japanese Patent No. 2,682,331 and Japanese Patent Application Laid-Open No. 6-138697 disclose, e.g., an Mg/Al-based toner incorporated with one or two types of divalent metals (e.g., Zn), a 3- or 4-element toner containing hydrotalcite, aimed at charging stability. However, they are still insufficient in charging stability and transferability under severer conditions.